NetBean IDE 5.5.1 become slow ?

Hi, All:
    Recently I'm using the Sun NetBean IDE 5.5.1 in Windows, and I found
it'll become very slowing when long time no operation then open it.
In task manager if no time to use the NetBean, the memory will become low;
then open NetBean it increase the memory and dif****ult to use the editor.
    Fianlly it up to near 150M, then operation is smooth, would you tell
me
what happend and how to avoid the memory decreasing ?

Regards. Ahan Hsieh